/**
 *   @FileName	a.cpp
 *   @Author	kanpurin
 *   @Created	2020.10.13 15:28:30
**/

#include "bits/stdc++.h" 
using namespace std; 
typedef long long ll;



struct StronglyConnectedComponents {
private:
    vector<vector<int>> g, rg;
    vector<int> order; 
    vector<bool> visited;
    int V;
    void dfs(int v) {
        if (visited[v]) return;
        visited[v] = true;
        for (int u : g[v]) dfs(u);
        order.push_back(v);
    }
    void rdfs(int v, int c) {
        if (comp[v] != -1) return;
        comp[v] = c;
        for (int u : rg[v]) rdfs(u, c);
    }
public:
    vector<vector<int>> Graph; 
    vector<int> comp; 
    StronglyConnectedComponents(int v) : V(v) {
        g.resize(v);
        rg.resize(v);
        visited.resize(v);
        comp.resize(v);
    }
    void add_edge(int from, int to) {
        g[from].push_back(to);
        rg[to].push_back(from);
    }
    void build() {
        Graph = vector<vector<int>>();
        order = vector<int>();
        visited.assign(V, false);
        comp.assign(V, -1);
        for (int i = 0; i < V; i++) dfs(i);
        reverse(order.begin(), order.end());
        int number = 0;
        for (int i = 0; i < V; i++) {
            if (comp[order[i]] == -1) {
                rdfs(order[i], number++);
            }
        }
        Graph.resize(number);
        for (int i = 0; i < V; i++) {
            for (int v : g[i]) {
                if (comp[i] == comp[v]) continue;
                Graph[comp[i]].push_back(comp[v]);
            }
        }
    }
};
int main() {
    int n;cin >> n;
    vector<pair<int,int>> p(n);
    int ans = 0;
    StronglyConnectedComponents g(n);
    for (int i = 0; i < n; i++) {
        cin >> p[i].first >> p[i].second;
        ans += p[i].first;
        g.add_edge(i,p[i].second-1);
    }
    g.build();
    int sz = g.Graph.size();
    vector<int> in(sz);
    for (int i = 0; i < sz; i++) {
        for(auto v : g.Graph[i]) {
            in[i]++;
        }
    }
    constexpr int INF = 1e9 + 6;
    vector<int> saishou(sz,INF);
    for (int i = 0; i < n; i++) {
        if (in[g.comp[i]] != 0) continue;
        saishou[g.comp[i]] = min(saishou[g.comp[i]],p[i].first);
    }
    for (int i = 0; i < sz; i++) {
        if (in[i] != 0) continue;
        ans += saishou[i];
    }
    printf("%.1f",(double)ans/2);
    return 0;
}